Traditionally, control of the tool holder locking is operated by a proximity switch associated with a disk rotating together with the tool holder. A signal from the proximity switch is generated in each of the angular positions into which the tool holder can be locked. The adjacent areas of the disk which operate the proximity switch are offset relative to one another around a relatively small angle of rotation, which is the case for turret heads with a great number of angular positions which can be selected. Any position may be selected from any other position. During operation at high switching velocity, as desired and sought for any such tool turret, the danger exists that the proximity switch will produce false signals.
According to one unpublished proposal, these difficulties can be avoided with the aid of a trigger cam arranged to revolve on a drive shaft for the tool holder. As a result of an intercalated gearing mechanism, the drive shaft also rotates at least 180.degree. when the tool holder is rotated into the directly adjacent angular position. With each revolution of this shaft, the trigger cam is carried along as a result of the friction lock as far as a next abutment. In the reversing operation required for the locking, the trigger cam is turned back into the switch position in which it triggers the control signal of the proximity switch.